30 DIPTERA OF NORTH AMERICA. [PART IV. 



organs of the female and adjusts thereon for copulation. The 

 structure of this outer forceps offers many modifications and is 

 for this reason very useful in the classification. 



The usual structure of the outer forceps is, that it consists of 

 two, generally subcylinclrical basal pieces, to each of which two 

 elongated, pointed, movable appendages are fastened (compare 

 Tab. IY, f. 23, 24, 25, 29). The two pairs of these appendages 

 are not of the same consistency, the outer one being generally 

 horny, the inner one often of a less hard texture. The modifica- 

 tions, however, of this primitive type are numerous. Sometimes 

 the two appendages on the same side are soldered together, so 

 as to represent a kind of horny hook (Tab. Ill, fig. 6, 1 ; Tab. 

 IY, fig. 11, 16). In the genus Dicranomyia the forceps is 

 represented by a pair of movable fleshy lobes, with horny, beak- 

 shaped projections on the inside (Tab. Ill, fig. 3, 5). Among 

 the Eriopterina the structure of the forceps is often complicated 

 and subject to considerable modifications (T. IY, f. 14, 15, 17-20). 

 The outer forceps, as far as I have been able to observe, is put 

 in motion by a kind of horny frame, fastened to its basis on the 

 inside and communicating with the proper muscles ; this frame 

 expands and contracts by means of a hinge in its middle (compare 

 Tab. IY, fig. 29, and 29 a, the forceps of Eriocera spinosa and 

 the explanation, appended to the figure ; I have observed a some- 

 what similar structure in Dicranoptycha sobrind). This inner 

 frame is also connected with the inner clutching apparatus, the 

 structure of which has not been used, however, for descriptive 

 purposes. Among the Limnobina, a single, immovable, styliform 

 organ is visible immediately below the forceps ; I have called it 

 the style ; this organ is not perceptible in most of the other sec- 

 tions. It is replaced, however, by a slender, horny, often curved 

 and pointed piece, which is entirely concealed when the forceps 

 is closed, and projected when it is open ; I have called it aculeus. 

 Among the Cylindrotomina, the aculeus has the shape of a lamel, 

 more or less trifid at the tip. (For more details on the structure 

 of the forceps, compare the explanation of Plates III and IY.) 

 A more detailed study of the structure of the male genitals and 

 also of the shape of the abdominal segments immediately pre- 

 ceding the forceps, would undoubtedly afford very valuable 

 characters for the discovery of links of relationship otherwise 



